In general, a power conversion device that performs DC-to-AC power conversion is provided with a DC/DC converter and an inverter. The DC/DC converter boosts a DC voltage of a DC power supply to a certain intermediate voltage and outputs the intermediate voltage to a DC bus, and the inverter converts the intermediate voltage to an AC voltage waveform. Here, the intermediate voltage is higher than the peak value (wave crest value) of the AC voltage. In order to cause the intermediate voltage to be a stable DC voltage, a large-capacitance capacitor of, for example, mF (milli-farad) level, is used as an intermediate capacitor connected to the DC bus. In a case of performing such power conversion, the DC/DC converter and the inverter constantly perform high-speed switching, and thus power loss such as switching loss arises correspondingly.
In order to reduce power losses and enhance efficiency, the applicant proposes a control method that compares the absolute value of the instantaneous value of an AC voltage to be generated and the DC voltage on the DC power supply side. A DC/DC converter should generate a DC waveform part for which the voltage needs to be stepped up and the inverter should generate an AC waveform part for which the voltage needs to be stepped down (see Patent Literature 1). In this control method, the DC/DC converter and the inverter alternately perform high-speed switching operations. Therefore, the DC/DC converter and the inverter alternately have stop periods in which high-speed switching is stopped, within one AC cycle. This method significantly reduces power losses. In this control method, the voltage of the DC bus becomes a voltage having a waveform like a pulsating current in which the waveform around the peak value of the AC voltage waveform is overlaid on the DC voltage. Therefore, as the intermediate capacitor connected to the DC bus, a small-capacitance capacitor (for example, several tens of μF) is used so as not to smooth the AC voltage waveform part.
As another reference, the following control method is also proposed: a pulsating current waveform as a base for the AC voltage waveform is generated by only switching of the DC/DC converter, and the inverter performs non-inversion or inversion of the polarity per one cycle of the pulsating current (see Patent Literature 2).